The problem of frost or ice formation on fins of such refrigerating systems is well known. There exists frost detecting devices which actuate a defrosting unit associated with the refrigeration apparatus by measuring a pressure differential or by measuring temperature; but none exists, however, which takes into consideration the thickness of frost or ice on the fins. The consideration of this thickness is important since it is at the beginning of frost formation that the refrigeration apparatus is most efficient. Hence, it would be extremely ill-timed to actuate the defrosting system when maximum efficiency is being performed.